The present invention generally relates to acceleration sensors having an inertial or "sensing" mass which moves in response to acceleration from a first position within a passage to a second position therein.
Known accelerometers used to control actuation of vehicle passenger safety restraints typically comprise a housing having a cylindrical passage formed therein; a spherical or cylindrical sensing mass located within the passage; a means for providing a return bias on the sensing mass, i.e., for nominally biasing the sensing mass to a first position within the passage; and a switch means mounted on the housing so as to be operated by the sensing mass when it moves in response to an acceleration input from its first position within the passage to a second position therein. Such accelerometers are typically of the "integrating" variety, i.e., the movement of the sensing mass within the passage is retarded through the use of friction damping, fluid damping or magnetic damping. See, e.g., U.S. Pat. No. 4,329,549 to Breed (gas damping through use of ball moving in closely-toleranced tube); U.S. Pat. No. 4,827,091 to Behr (magnetic damping through use of a magnetic sensing mass in combination with encompassing conductive, nonmagnetic rings).
Such known accelerometers work well when experiencing acceleration inputs which are coincident with the sensing axis thereof, i.e., the axis of the cylinder defining the passage in which the sensing mass moves. Thus, where the sensing axis of the accelerometer is aligned with the longitudinal axis of a motor vehicle, the accelerometer is most useful in detecting a "head-on" impact.
Correlatively, however, such known accelerometers are less suitable for use in detecting so-called "off-axis" impacts. Specifically, when the vehicle experiences an acceleration input along an impact axis which forms an impact angle .theta. relative to the accelerometer's sensing axis, the resultant force acting on the accelerometer's sensing mass along the sensing axis is significantly reduced, with an attendant reduction in the degree of passenger protection afforded by a restraint system controlled by the accelerometer. Stated another way, the accelerating force A.sub.x exerted on the mass in an off-axis impact is merely a component of the applied accelerating force A as projected upon the sensing axis, with a further retarding frictional load F which is itself proportional to the normal reaction component N of the applied accelerating force A. The effect may be summarized using the following equation: ##EQU1## Thus, for a given acceleration input A applied to the vehicle at a relative impact angle .theta. of, say, thirty degrees (i.e., where the acceleration input is applied thirty degrees off of the sensing axis of the accelerometer) and a coefficient of sliding friction .mu. of 0.20, the resulting acceleration force A.sub.x exerted upon the mass is only 76.6 percent of the applied acceleration input A. The end result is an effective increase in the triggering threshold of the accelerometer in the event the vehicle experiences off-axis acceleration inputs, with a corresponding reduction in passenger safety.
This distortion of the accelerometer's threshold in the event of off-axis impacts can be reduced by setting the side walls at an angle .phi.. The effect may be summarized using the following equation:. ##EQU2## Thus, if an accelerometer is provided with a passage having an eight degree side-wall angle and a coefficient of sliding friction .mu. of 0.20, the application of an acceleration input A at a relative impact angle .theta. of thirty degrees produces an accelerating force A.sub.x on the sensing mass which is approximately 84.4 percent of the applied acceleration input A--a substantial improvement over the 76.6 percent figure calculated above with respect to parallel-walled accelerometers. Indeed, evaluation of the above equation indicates that the percent increase in transmitted acceleration from off-axis impacts is roughly equal to the side-wall angle .phi. in degrees.
Accordingly, the prior art teaches accelerometers having angled side walls to accommodate off-axis impacts. For example, U.S. Pat. No. 3,774,128 to Orlando teaches an accelerometer featuring a ball-shaped sensing mass which travels within a horizontally-flared passage, i.e., within a passage having diverging side walls, in response to an acceleration input directed within the included angles of the passage's side walls. Specifically, the ball-shaped sensing mass is biased to a "ball seat" or rest position within the passage by a permanent magnet. An planar ferritic exterior bracket provides a suitable flux path for the magnetic return bias while further exerting a downward bias on the sensing mass to limit bouncing.
Unfortunately, however, the use of angled side walls in an accelerometer is not a panacea: while such accelerometers suffer from less distortion of their firing thresholds in the event of off-axis impacts, accelerometers such as the one taught by Orlando must necessarily be characterized as being of the nonintegrating type, inasmuch as they lack sufficient means for damping the movement of the sensing mass within the passage due to its changing cross-sectional dimensions. Moreover, where such accelerometers employ a magnetic return bias, as the side wall angle increases, increasingly complex magnetic circuits are required to ensure useful force-versus-displacement curves for all included angles, with an ultimate limit as to side wall angle .phi.. Still further, the use of angled side walls presents problems relating to contact design and achievable contact dwell, particularly where multiple circuit contacts are desired; and the additional degree of freedom (yaw) can be a disadvantage in controlling system dynamics and the contacts interface. Finally, known accelerometers having angled side walls are more difficult to manufacture than their parallel-walled counterparts.
Therefore, what is desired is an integrating accelerometer having angled side walls and featuring nearly identical return-bias-force-versus-displacement curves for sensing mass displacement along all included angles, increased contact dwell, and multiple circuit capability, as well as featuring improved testability and reconfigurability functions.